(1) Field of the Invention
The present invention relates to a resin composition for sealing semiconductors. The composition has a low elastic modulus, a low heat expansion coefficient, a high resistance to heat and a high resistance to thermal shock. Accordingly, the composition is particularly suitable for sealing electronic parts, such as semiconductors, where high reliability is required.
(2) Description of the Prior Art
Recently, so-called plastic sealing using thermosetting plastics, such as epoxy resins, has been widely commercialized to seal semiconductors. This is due to its economical merits, for example, relatively cheap material cost and ease of mass production. One particular resin composition used for this purpose comprises a polyfunctional epoxy resin, a phenol novolac resin and an inorganic filler. The composition is characterized by its high heat resistance, good processability and excellent electrical properties.
As the high integration of semiconductor chips has advanced, the size of the chips have become larger. On the other hand, as the high density assembly of semiconductors to a substrate has progressed, the shape of chip-containing packages have become smaller and thinner, such as a flat package.
The demand for packages in the art has caused some failures which have heretofore not been observed using conventional sealing resins. The stress caused by the heat expansion coefficient differences between the sealing resin and the chip, due to increased chip size and the decrease of resin layer thickness is believed to have led to cracking of the passivation film or cracking of the sealing resin composition by thermal shock. This cracking decreases the humidity resistance of the semiconductor and results in low reliability of the semiconductor. Therefore, it has been desired to develop a sealing resin which has decreased stress.
One way to decrease the stress is by reducing the heat expansion coefficient of the resin to reduce the rate difference between the heat expansion rate of the resin and that of the chip. However, the rate difference between the resin and the chip is generally so large that to lessen the rate difference, it is necessary to incorporate a large amount of inorganic filler which has a lower heat expansion coefficient. This type of inorganic filler has already been used for this purpose in sealing resins, so raising the amount of the filler leads to unacceptable processability.
Another way to decrease the stress is by reducing the elastic modulus of the resin. For this purpose, attempts have been made to add a plasticizer into the resin, to use a pliable epoxy resin or to use a phenolic resin. However, the cured resins obtained by such attempts have insufficient heat resistance.
As represented by the Japanese Laid Open Patent (Tokkyo Kokai Koho) No. 58-108220, it was proposed to maintain the heat resistance of the resin by dispersing rubber particles in the sealing resin to give the resin crack-resistant properties. However, this method provides certain problems such as resin deposits on the mold or low heat shock resistance at temperatures above the glass transition temperature which typically are encountered in a bath of molten solder. The low heat shock resistance results in a decrease of the reliability of the semiconductor after dipping in the bath of molten solder which is a fatal defect for sealing materials used for integrated circuits.
Additionally, if the degree of deposit on the mold is large, it is necessary to clean the mold frequently which leads to low productivity and reduced economy.